The invention relates to a ceramic filter element, in particular a filter candle or filter tube. The invention also relates to a method for the manufacture of such filter elements.
The reduction of nitrous oxides from gases is carried out by selective reaction of the nitrous oxides with the reduction media in solid catalytic converters. In order for an adequate catalytic activity to be attained, and in order to exclude the depositing of salt on the surface of the catalyst, so leading to its deactivation, correspondingly high gas temperatures are required. For example, reduction catalytic converters are used for waste gas purification in power stations in the waste gas flow between the boiler and the air pre-heater at temperatures of some 300° C. to 350° C.
The ceramic catalyst material is in this situation arranged in most cases in the form of rigid plates or honeycomb bodies, but in part also in the form of bulk deposit layers.
With what is referred to as the Low-Dust method, located upstream of the honeycomb body or plate catalytic converter is a hot-gas electric filter for substantial separation of the dust. With what is referred to as the High-Dust method, the gas passes the reactor, filled with a catalyst, without prior dust extraction. The individual catalyst elements must be provided with correspondingly large free flow channels for the gas, in order to avoid blockages.
The method variants mentioned have the feature in common that, for the nitrous oxide separation, in addition to the apparatus and devices already present, further large-volume and elaborate apparatus is added for the precipitation of other contaminant and pollutant components. An elaborate gas purification technique of this type is particularly disadvantageous if the treatment of comparatively small gas flows is concerned.
It is therefore proposed in DE 36 34 360 that the gases containing nitrous oxides are mixed with the reaction medium and that the mixture is conducted through at least one filter element having catalytic effect, which consists of a ceramic carrier material and the catalytic effect substances. The catalytic effect filter element is also used for extracting dust from the gases containing nitrous oxides. The filter element consists either of felts or non-woven fleece materials, which are manufactured from fibres of the ceramic carrier material by compaction and have been doped with the catalytic effect substances, or of porous sinter bodies, which are manufactured from granules of the ceramic carrier material by sintering and have been doped with the catalytic effect substances. The filter element may exhibit the form of a filter candle or filter element. The doping of the ceramic carrier material in the catalytic effect substances is effected by these substances being applied onto the ceramic carrier material before, during, or after the manufacture of the felts, non-woven fleeces, or sinter bodies. This can be done, for example, by impregnating the filter elements with salt solution and subsequent heating of the filter elements doped with the salts.
From DE 37 05 793 a filter device for gas purification is known which exhibits a single tubular body made of heat-resistant foam ceramics, which simultaneously has the effect of a dust filter and, with the appropriate coating, as a catalytic converter.
In EP 0 470 659 a method is described for the separation of dust and organic compounds from oxygen-containing gases, in particular from combustion waste gases. In this situation, the waste gas which is to be purified is conducted through at least one filter element functioning as a catalytic converter, which is composed of a ceramic carrier material and the substances functioning as catalysts. The filter element may consist of a porous sinter body, which is manufactured from granules of ceramic carrier material by sintering and has been doped with the catalytically effective substances.
WO 9012950 describes a Diesel soot filter, which consists of honeycomb bodies, of which the first honeycomb body is provided with a first surface layer which, in an inherently known manner, has the effect of catalytically converting the nitrous oxides and carbon monoxide into nitrogen or carbon dioxide respectively, while the second honeycomb bodies are provided with a second surface layer and in an inherently known manner catalytically cause a reduction of the ignition temperature of the soot adhering here.
From WO 9803249 a gas purification device in the form of a filter candle is known. The filter candle exhibits on the outside a membrane layer of ultra-fine silicon carbide particles, in order to filter out dust particles. A catalytically effective layer of sintered silicon carbide powder follows in the direction of flow inwards. As the catalytic converter, a vanadium-titanium compound is provided for, with which the silicon carbide filter is impregnated. The filter candle is created by a subsequent coating of a porous element, which has the disadvantage that the porous layer cannot be applied entirely uniformly in the pore area, and therefore the distribution of the catalyst material is not homogenous. In addition to this, there is the problem of the adherence of the applied layer to the silicon carbide particles, whereby an added difficulty is that, in the area of the hot-gas filtration, temperature fluctuation stresses occur, which can encourage the separation of the coating.
It is therefore the problem of the invention to provide a ceramic filter element which can be used for hot-gas filtration, exhibits a high degree of efficiency, and with which the risk does not arise of the catalytically effective components become detached from the filter.
This problem is resolved with a ceramic filter element which, according to a first embodiment, is characterised in that the bonding material exhibits catalyst material or is at least in part replaced by catalyst material, and that the support material particles are connected to one another by means of the catalyst material and/or the bonding material.